Keeping A Breast Of Those Sayings

Origin of "colder than a witches tit"

The simple explanation is that "colder than a witch's tit" is just a vivid metaphor, like "hotter than the hinges of hell." Since a witch is in league with Satan, presumably she has no maternal feelings. Thus the medium by which she would suckle a child is, well, cold as a witch's tit.

But there's some history behind this wisecrack. A witch's tit (or witch's teat, to use the older spelling) supposedly left a marking that witch hunters and courts would look for on the body of an accused person. Supposedly, witches would suckle their familiars, and sometimes the Devil himself, from this "unholy" body part. To find these marks, as well as insensitive spots on the skin called devil's marks--caused by the Devil's claws or teeth--the suspects were stripped, shaven, then closely examined for any blemishes, moles, or even scars that could be labeled as diabolical. To find marks invisible to the eye, the examiner would
poke the victim inch by inch with a blunt needle (called a bodkin) until they found a spot that didn't feel pain or bled. Discovery of these marks or spots--one supposes they would be considered cold since they were a sign of communion with the
Devil--would be "proof" of the person's dealings with Scratch, so they would be shown in full court before the execution.

Source: The Phrase Finder.